There are known in the art catalytic systems of the Ziegler-Natta type, active in the polymerization of alpha-olefins, generally consisting of the combination of an organometal compound of elements of groups I-III and of a compound of a transition metal belonging to groups from IV to VI of the Periodic table (Boor Jr., "Ziegler-Natta Catalysts and Polymerization", Academic, New York, 1979). Generally an aluminum alkyl is used as the organometallic compound and a titanium halide is used as the transition metal compound. The possibility of binding or depositing said titanium halide on a solid and granular carrier is also known (Karol F. J., Catal. Rev.-Sci. EngN, 26, 384, 557-595, (1984)).
The activation of the magnesium chloride and its use for preparing catalysts comprising titanium salts, highly active for polymerizing olefins is also known, for instance as described in DE 2 153 520 (CA 77,62505,1972); DE 2 638 429 (CA 83,59870,1972); DE 2 638 429 (CA 86,140706,1975); Be 848 427 (CA 87, 68839,1977); and JP 79/118 484 (CA 92, 59450,1979). Finally, it is known that heterogeneous catalysts for the polymerization of alpha-olefins are able to replicate their morphology in an equivalent polymeric morphology; this is made possible by carrying out complicated technological conditions for the synthesis of the catalyst (Karol F. J., above mentioned, and McDaniel M. P., J. Polym. Sci., Polym. Chem. Ed., 19,1967-1976,1981).
The market for polyolefins requires products which are more and more suitable to satisfy different application requirements, whereas there is also the requirement of simplifying the production processes for polyolefins, in order to reduce investment and production costs. In particular, there is felt the requirement to have catalysts, which are not only simple and inexpensive, but are suitable for producing if possible in one single polymerization step olefinic homoplymers and copolymers suitable either for extrusion or molding.
Eaborn C. E. in "Organo Silicon Compounds; Butterworths Scientific Publications, London 1960; Rochow E. G. in "The Chemistry of Silicon" New York, 1975; and Voorhoeve R. J. H. in "Organosilanes", Elsevier, New York, 1967, describe an alkylation reaction between a dialkylmagnesium, or an alkylmagnesium halide and silicon tetrachloride, which gives yield to a non crystalline solid compound. M. P. McDaniel, J. Catal., 76, 17 (1982); M. P. McDaniel, J. Phys. Chem., 85, 532 (1981); C. G. Armistead et al., J. Phys. Chem., 73/II, 3947 (1969); R. J. Peylar et al., J. Catal., 20, 309 (1971); J. Kunawicz et al., Trans. Faraday Soc. 67, 3848 (1971) describe the adsorption of a magnesium dialkyl or a magnesium alkyl chloride, as well as of halogenated compounds, such as silicon tetrachloride and titanium tetrachloride, on a silica or other inert carrier containing hydroxyl groups.
It has now found according to the present invention, that a dialkylmagnesium or an alkylmagnesium halide, are able to interact with a silicon chloride on a solid and porous carrier on which a compound of a transition metal, such as hafnium, zirconium or vanadium has been previously adsorbed. It has also been found that the solid product of this reaction is able to interact with a titanium compound to give a solid component of catalyst highly active for the polymerization of ethylene and for the copolymerization of ethylene with alpha-olefins. Thus, according to the present invention it is possible to obtain in a simple and cheap way solid components of catalysts, highly active in the (co)polymerization of ethylene in processes working in suspension or in gaseos phase, in one single step, and which are able to produce ethylene polymers having the desired density values, a distribution of the molecular weight from middle to wide and a monomodal distribution of the molecular weights.
The present invention relates to a process for the preparation of a solid component of a catalyst for the (co)polymerization of ethylene, containing magnesium chloride, silicon, titanium and at least another metal selected from hafnium, zirconium and vanadium, on a granular solid carrier, said process comprising:
(i) adsorbing a compound of a metal M selected from hafnium, zirconium or vanadium on a granular and porous solid carrier by contacting this carrier with a solution of the metal M, in an organic solvent, followed by evaporation of the solvent;
(ii) impregnating the solid carrier treated in step (i) by contacting it with a solution, in an organic solvent, of a dialkyl magnesium or alkylmagnesium halide, followed by evaporation of the solvent;
(iii) contacting the solid carrier treated in step (ii) with a silicon chloride; and
(iv) contacting the solid carrier treated in step (iii) with a titanium compound to give a solid component of catalyst;
Wherein the quantity of the carrier in said solid component of catalyst is 30 to 90% by weight, the atomic ratios among titanium, metal M, magnesium and silicon in the reagents being within the following ranges: Ti:M:Mg:Si=1:0.1-3:1-20:0.1-50.